In the production of lubricating oils from crude petroleum stocks, the crude oil is fractionated to obtain a product wherein at least 90 volume percent of such product boils in the lubricating oil range i.e. above about 650.degree. F. (343.degree. C.). As employed throughout this specification, this fraction obtainable by the atmospheric distillation of a crude oil will be termed a "crude lubricating oil". Traditionally, this crude lubricating oil is subjected to a further distillation, conducted at less than atmospheric pressure, to obtain a distillate fraction and a residual lubricating oil fraction. The residual lubricating oil fraction is then subjected to deasphalting such as by treatment with a light hydrocarbon solvent, e.g. propane.
The substantially asphaltic-free residual lubricating oil and distillate lubricating oil fractions are then independently subjected to a processing step to reduce the aromatic content thereof and to increase the viscosity index (V. I.). This processing step generally comprises contacting the lubricating oil feed with a solvent selective for aromatics.
The raffinate or semi-refined lubricating oil obtained from the solvent extraction step is dewaxed to lower the pour point to a desired level. Finally, the dewaxed oil is subjected to a finishing operation to make small adjustments in the characteristics in the oil. Conventional finishing operations include a mild hydrogen treatment, acid contacting and clay contacting wherein minor concentrations of contaminants such as sulfur are removed.
When the above-described lubricating oil manufacturing process is employed for crude lubricating oils containing significant concentrations of sulfur, yields of finished lubricating oil base stocks are substantially reduced and processing requirements for dewaxing and finishing the processed lubricating oils are more severe with a resultant substantial increase in the costs of manufacture. Additionally, the high sulfur concentration of the extract fraction recovered from the solvent extraction step makes it undesirable to use the extract conventionally in the production of low sulfur fuel oils.